In recent years, considerable efforts have been made to reduce the level of hydrocarbon (HC) emissions from vehicle engines. Conventional exhaust treatment catalysts such as three-way catalysts achieve oxidation of hydrocarbons to CO2 and water and help prevent the exit of unburnt or partially burnt hydrocarbon emissions from a vehicle. However, these emissions are high during cold starting of the engine before the latent heat of the exhaust gas allows the catalyst to become active (e.g., achieve light-off temperatures).
Attempts to reduce emissions during engine cold-start may include providing an auxiliary heating device to heat the catalyst to light-off temperatures. Other attempts to address engine cold-start emissions include integrating a hydrocarbon trap (HC trap) with a catalyst. One example approach is shown by Jen et al. in U.S. 20130287659. Therein, the HC trap system is positioned in an exhaust passage. The HC trap captures HCs released during the cold-start and retains the HCs until the catalytic elements are sufficiently heated to reduce the trapped HCs.
However, the inventors herein have recognized potential issues with such systems. As one example, by placing the HC trap system in the exhaust passage, it is continually exposed to exhaust gas and combustion byproducts, which may decrease the longevity of the HC trap. Furthermore, oxidation of HCs in the presence of water is inefficient compared to oxidation in the absence of water. Additionally, the HC trap may warm-up inefficiently due to thermal losses in the exhaust pipe. As another example, as the HC trap temperature increases, the HC trap may release 50% or more of the trapped HCs before the catalyst system reaches the light-off temperature. This issue is further exacerbated as the HC trap releases HCs at a lower temperature and the catalyst lights-off at higher temperatures over continued use during a life of the HC trap. As such, the HC trap system may not sufficiently reduce emissions during cold-starts.
In one example, the issues described above may be addressed by a method comprising flowing combusted exhaust gas to a particulate filter and hydrocarbon trap in a bypass during a cold start, the particulate filter positioned also inside the a main exhaust passage and the hydrocarbon trap positioned outside the main passage. In this way, the HC trap may decrease emissions during a cold-start by capturing untreated emissions passing through a catalyst upstream of the bypass.
As one example, the main exhaust passage is larger than the bypass, and as such, HC trap a diameter of the hydrocarbon trap is less than a diameter of the main exhaust passage. The hydrocarbon trap is located in a portion of the bypass passage spaced away from the main exhaust passage such that a gap is located between exterior surfaces of the main exhaust passage and the bypass. A diverter valve is located in the main exhaust passage between an inlet and an outlet of the bypass. As such, the diverter valve may be actuated to adjust exhaust gas flow through the inlet of the bypass. When the diverter valve is closed, exhaust gas backpressure increases and exhaust gas is forced to flow from the main exhaust passage to the bypass. As an example, the diverter valve is moved to a more closed position during cold-starts where the catalyst of the main exhaust passage is not lit-off. In this way, cold-start emissions flowing through the inactive catalyst may be captured and treated by the hydrocarbon trap in the bypass. By placing the hydrocarbon trap in the bypass, exhaust gas flow to the bypass may decrease following activation (e.g., light-off) of the catalyst by actuating the diverter valve to a more open position. This may increase a lifespan of the hydrocarbon trap.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.